Future-proof optical telecommunications networks have to satisfy stringent requirements relating to capacity and flexibility. Such requirements are optimally satisfied by transmission and switching using optical frequency-division multiplex (wavelength-division multiplex WDM) wavelength-division multiplex allows the capacity of optical transmission networks to be considerably increased; and WDM coupling arrangements (Optical Cross Connects OCC) allow the flexibility of such networks to be enhanced. In order to achieve the high performance of such networks with a complexity level which is as low as possible at the same time, it should be possible to transmit the optical signals as far as possible without (electrooptical) regeneration. The use of fiber amplifiers allows the attenuation of optical signals on optical conductors and in WDM switching matrices to be compensated for, so that the length of a section which is free of regenerators is in principle not limited by attenuation in any case.
However, problems arise in wavelength-division multiplex systems from the fact that the individual WDM channels are attenuated or amplified to different extents. Minor differences are caused just by tolerances in the individual system components (such as fiber amplifiers, optical fibers, connectors, WDM switching matrices); and in the case of long sections without regenerators, these can accumulate so as to result in level differences occurring which prevent clean separation of the wavelength-division multiplex channels. It is thus desirable to be able to insert into the network components which reduce these level differences.
At a normal ambient temperature (owing to homogeneous linear propagation at a normal ambient temperature), such level differences cannot be reduced by means of a fiber amplifier. It is admittedly known that fiber amplifiers have non-homogeneous linear propagation at low temperatures (77 K) (IEEE Photonics Technology Letters, 2 (1990), p. 246-248; OFC/IOOC""93 Technical Digest, p. 174-175); however, the amount of cooling required in this case prevents practical use.
In principle, it is known (from IEEE Photonics Technology Letters, 8 (1994) 11, p. 1321-1323) for level regulation to be carried out by physically separating the individual WDM channels from one another using a WDM demultiplexer, and for the optical signals in each channel to be amplified in their own right by means of a channel-specific fiber amplifier operated in the saturation region, after which the signals are once again combined in wavelength-division multiplex form by means of a WDM multiplexer. Level regulation for a fixed wavelength scheme can thus be achieved by using channel-specific optical components, a correspondingly large number of which therefore have to be provided.
In contrast, the invention indicates a different way to achieve level regulation.
The invention relates to a circuit arrangement for the operation of a wavelength-division multiplex system; this circuit arrangement is characterized, according to the invention, in that, on an acousto-optical add/drop multiplexer to whose input the wavelength-division multiplex signal is applied, a plurality of surface acoustic waves are excited at specific, different frequencies and a portion of the light power is separated from wavelength channels at an optical frequency which is governed by the frequency of such a surface acoustic wave, and in that a superheterodyne receiver is provided to whose input side the separated light power elements are applied as well as light power elements which are output from the non-separated elements of the wavelength-division multiplex signal in a directional coupler which is connected downstream of the acousto-optical add/drop. multiplexer, and which superheterodyne receiver emits an output signal which corresponds to the light power levels which are transmitted in a channel-specific manner in the individual wavelength channels.
Such a circuit arrangement provides the capability for a deliberate determination, which can be flexibly matched to a wavelength scheme, of the optical levels which occur in each case in a plurality of wavelength channels, using only one acousto-optical add/drop multiplexer. In a further refinement of the invention for this purpose, it is possible to provide electrical mixing of the electrical signals which excite the surface acoustic waves at different frequency and the electrical output signal of the superheterodyne receiver in order to use the output signal of the superheterodyne receiver to obtain the channel-specific individual signals which correspond to the light power levels which are transmitted in a channel-specific manner in the individual wavelength channels.
If, according to a further invention, channel-specific control of the intensityxe2x80x94which governs the proportion of the separated light power in the individual wavelength channelsxe2x80x94of the surface acoustic waves is provided, then this advantageouslyxe2x80x94once again using only one acousto-optical add/drop multiplexerxe2x80x94also allows channel-specific attenuation of the individual wavelength channels, in which case the relevant WDM channels are determined on the basis of the respective optical wavelength, in accordance with the frequency of the acoustic waves.
In order to make it possible to avoid undesirable level differences throughout the entire WDM system, it is possible, in a further refinement of the invention, to connect downstream from the superheterodyne receiver a regulating device for in each case separately readjusting the light power transmitted in the individual wavelength channels to a value predetermined for the respective wavelength channel. In this case, in a further refinement of the invention, it is possible, by electrically mixing the electrical signals which excite the surface acoustic waves at a different frequency and the electrical output signal of the superheterodyne receiver, for the regulating device to separate from one another the signals which are contained therein and correspond to the light power transmitted in the individual wavelength channels and thus, in a channel-specific manner, to control the intensity of the electrical signals which excite the surface acoustic waves.
In conjunction with a regulated fiber amplifier (or else a plurality of regulated fiber amplifiers), this allows there to be long distances between (electrooptical) regenerators in the wavelength-division multiplex system.
At least the purely optical components of the superheterodyne receiver can, according to a further irvention, be integrated on a substrate together with the acousto-optical add/drop multiplexer, which advantageously makes it possible to provide the optical functions of level measurement and level adjustment using a single acousto-optical component. The invention advantageously requires no optical components to customize the individual wavelength channels.
Further special features of the invention will become evident from the following more detailed explanation of the invention, with reference to the drawings.